The present invention relates to alignment notches in a buried heterostructure edge emitting laser for alignment of the device in a passive manner.
The present invention is related to U.S. patent application Ser. No. 09/031,586 to Imhoff, filed Feb. 27, 1998 now U.S. Pat. No. 5,981,975 as well as to U.S. patent application Ser. No. 60/079,909 filed on Mar. 30, 1998, the disclosures of which are specifically incorporated herein by reference. Light emitting devices often utilize double heterostructures or multi-quantum well structures in which an active region of a III-V semiconductor is sandwiched between two oppositely doped III-IV compounds. By choosing appropriate materials for the outer layers, the band gaps are made to be larger than that of the active layer. This procedure, well known to one of ordinary skill in the art, produces a device that permits light emission due to recombination in the active region, but prevents the flow of electrons or holes between the active layer and the higher band gap sandwiching layers due to the differences between the conduction band energies and the valence band energies, respectively. Light emitting devices can be fabricated to emit from the edge of the active layer, or from the surface. Typically, a first layer of material, the substrate, is n-type indium phosphide (InP) with an n-type buffer layer disposed thereon. This buffer layer again is preferably InP. The active layer is typically indium gallium arsenide phosphide (InGaAsP) with a p-type cladding layer of InP disposed thereon. One potential pitfall of double heterostructure lasers is often a lack of means for confining the current and the radiation emission in the lateral direction. The result is that a typical broad area laser can support more than one transverse mode, resulting in unacceptable mode hopping as well as spacial and temperature instabilities. To overcome these problems, modern semiconductor lasers employ some form of transverse optical and carrier confinement. A typical structure to effect lateral confinement is the buried heterostructure laser. The buffer, active and cladding layers are disposed on the substrate by epitaxial techniques. The structure is then etched through a mask down to the substrate level leaving a relatively narrow (roughly on the order of several microns) rectangular mesa composed of the original layers. A burying layer is then regrown on either side of the mesa resulting in the buried heterostructure device. The important feature of a buried heterostructure laser is that the active layer is surrounded on all sides by a lower index material so that from an electromagnetic perspective the structure is that of a rectangular dielectric waveguide. The lateral and transverse dimensions of the active region and the index discontinuities are chosen so that only the lowest order transverse mode can propagate in the waveguide. Another very important feature of the structure and that which is required to effect lasing is the confinement of injected carriers at the boundaries of the active region due to the energy band discontinuities at the interface of the active region and the InP layers. These act as potential barriers inhibiting carrier escape out of the active region.
One area of optoelectronics which has seen a great deal of activity in the recent past is in the area of passive alignment. Silicon waferboard, which utilizes the crystalline properties of silicon for aligning optical fibers, as well as passive and active optical devices, has gained a great deal of acceptance. One technique for aligning an optoelectronic device to an optical fiber and other passive and/or active elements is the use of an alignment pedestal for lateral planar registration and standoffs for height registration. By virtue of the sub-micron accuracy of photolithography used to define and align these pedestals and standoff features, the application of this approach has proven to be a viable alignment alternative. By effecting alignment in a passive manner, the labor input into the finished product can be reduced, resulting in lower cost of the final product. One example of such an alignment scheme can be found in U.S. Pat. No. 5,163,108 to Armiento, et al., the disclosure of which is specifically incorporated herein by reference. The reference to Armiento, et al. makes use of an alignment notch on the active device which is designed to mate with alignment pedestals and standoffs on the silicon waferboard. This particular structure is used for aligning an optical fiber array to an array of light emitting devices.
Unfortunately, one problem with structures like the one shown in the reference to Armiento, et al., is that the alignment notch in the laser structure is limited to a ridge laser structure. This is because in a ridge waveguide laser structure, the patterning photolithography step which defines the active waveguide is simultaneously used to define the alignment notch in the same mask level, resulting in a alignment of the notch and active waveguide which is limited to a ridge laser structure. However, it is advantageous from a performance standpoint, to be able to utilize lasers and other active devices which incorporate a regrowth step, as described above. For this class of devices, the subsequent regrowth step(s), which bury the active waveguide mesa and notch create a more complicating fabrication sequences. The fabrication of an alignment notch after the regrowth is needed since the notch pattering step must occur in a photolithography step that defined the notch and the active at the same time. Moreover, a notch pattering step on the regrown surface of the wafer is difficult because the mesa is not a visible re-alignment feature using the conventional technique of optical alignment methods. Accordingly, what is needed is a technique for effecting an alignment fiducial, such as a notch, in a buried double heterostructure laser or other buried heterostructure device which is accurately aligned to the optical axis of the emitted light beam.
The present invention relates to a buried heterostructure laser with a novel alignment notch on either side of the laser die, with the notch formed by a quaternary (Q) layer of InGaAsP which is used to effect alignment of the die in a passive manner on silicon waferboard. The Q layer on the side mesa is an etch stop layer and effects alignment as the notch has a side surface for alignment against a pedestal (x-direction alignment in FIG. 2) and a top surface of the Q etch stop layer effects alignment against a standoff (y-direction in FIG. 2).